1. Field of the Invention
The present invention relates to a spread spectrum pulse position modulation communication system (for example, for use in indoor radio communication, radio LAN, radio high-speed data communication, etc.).
2. Discussion of the Background
The principle of spread spectrum pulse position modulation is shown in an English paper, Spread Spectrum Pulse Position Modulation written by Isao Okazaki and Takaaki Hasegawa in IEICE TRANS. COMMUN., VOL. E76-B., NO. 8, August, 1993, pages 929-940. The teaching of the paper is hereby incorporated by reference.
With regard to spread spectrum pulse position modulation, Isao Okazaki, Takaaki Hasegawa and Saitama University wrote a Japanese paper entitled A Study on Multiplexing of Spread-Spectrum Pulse Position Modulation in SST91-18, pages 17-22.
Japanese Laid-Open Patent Application No. 8-79133 and the corresponding U.S. Pat. No. 5,596,601 (hereinafter "the '601 patent") of AT & T CORP disclose two-signal multiplexing by quadrature modulation.
The inventor of this application is the same as the inventor of U.S. application Ser. No. 08/862,647 (hereinafter "the '647 application") filed on May 23, 1997 now U.S. Pat. No. 5,923,701 which discloses two-signal multiplexing as a result of adding together a spread spectrum pulse position modulation signal with a pseudonoise code and a spread spectrum pulse position modulation signal with the inverted pseudonoise code.
A spread spectrum pulse position modulation communication system in the related art will now be described with reference to FIGS. 1A, 1B, 1C and 1D.
FIG. 1A shows a modulated signal in a case of simple pulse position modulation and shows an example where 4 slots are provided for each frame. For an M value data symbol to be transmitted, one of M slots is selected and a pulse is transmitted. Thus, a pulse position modulation is performed.
FIG. 1B shows a modulated signal of the spread spectrum pulse position modulation communication system which is a system resulting from combining a spread spectrum modulation with the system shown in FIG. 1A. As described in Japanese Laid-Open Patent Application No. 4-137835, in this system, instead of one slot width of a pulse in the pulse position modulation in the related art, a code length L of a pseudonoise code is inserted into L slots starting from a selected slot. Thus, spread modulation is performed. In order to prevent overlapping of signals between adjacent frames, a frame length is longer by more than L-1 slots as compared to the pulse position modulation. Accordingly, the number of slots for each frame is M+L-1+j. When j.gtoreq.0, signals are not overlapped. When j&lt;0, some overlapping of signals occurs.
In the example of FIG. 1B, one of the M slots starting from the top to be transmitted is selected to correspond to data obtained from differential encoding. The pseudonoise code is inserted into the L slots starting from the selected slot. Thus, spread modulation is performed. In this example, FIG. 1B shows a transmission signal in a case of M=4, L=7 and j=0, and shows a modulated signal in a case where data obtained from differential encoding to be transmitted is 0, 1, 3, etc.
The signal shown in FIG. 1B is input to a matched filter which matches the code the same as the pseudonoise code used in the spread modulation. As a result, a pulse position modulated signal shown in FIG. 1C is reproduced. This is because the autocorrelation characteristics of the pseudonoise code used in the spread modulation are such that, as shown in FIG. 1D, a sharp peak occurs only when a time difference between codes is within one slot period. Then, by obtaining the position of the slot position of the reproduced pulse in each frame, the original data can be reproduced.
FIGS. 2 and 3 show circuit arrangements of a transmitter and a receiver which concretely realize the above-described processes. In the transmitter shown in FIG. 2, a clock signal generator 1 drives (1) a pseudonoise code generator code 9 and (2) a counter 2 (which returns to zero each time (M+L-1+j) pulses are counted). Serial data to be transmitted is converted into a parallel data through a serial-parallel converter 5. Parallel data of one frame before is stored in a register 8, the output value of the register 8 is added to the parallel data from the serial-parallel converter 5 through an adder 6. The output of the adder 6 is fed back to the register 8. Thus, differential encoding is performed. The output value of the register 8 is compared with the value of the counter 2 by a comparator 4. When the values agree, the comparator 4 sends a trigger pulse signal to the pseudonoise code generator 9. Thereby, the pseudonoise code generator 9 generates one period of a pseudonoise code. A detector 3 which detects that the output of the counter 2 becomes a predetermined value generates a frame clock signal. The register 8 operates in synchronization with the frame clock signal. Further, the frequency of this clock signal is multiplied by a PLL 7 or the like, and the resulting clock signal is used in the serial-parallel conversion. The signal from the pseudonoise code generator 9 is multiplied by the signal from an oscillator 11 through a multiplier 10, and thus, is converted into a high-frequency signal. The high-frequency signal passes through a filter 12 and is transmitted as a radio signal through an antenna.
FIG. 3 illustrates the reception portion. In the reception portion, the signal from the transmission portion is received by an antenna and is amplified by an amplifier 20. Then, the thus-obtained signal is multiplied by a local oscillation signal from an oscillator 22 through a multiplier 21. Thereby, the signal is converted into an intermediate frequency signal. This signal passes through a filter 23 and is amplified by a gain controlled amplifier 24. Then, the signal passes through a matched filter 25 which uses the same pseudonoise code as that of the transmission portion. Thereby, inverse spreading is performed and a pulse position modulated signal is reproduced. Detection is performed by a detector 26 at the output of the filter 25, and the signal is converted into a baseband pulse-position modulated signal. Pulse intervals of this signal are measured by a following pulse interval measuring circuit 27. Transmitted data is reproduced from the measured value, and finally, the data is converted into serial data by a parallel-serial converter 28. Thus, the originally transmitted signal is reproduced. In the above-described system in the related art, only the amplitudes of matched pulses are seen.